This R21 application seeks to determine if infection-sensiztied perinatal hypoxic-ischemic injury promotes monocyte invasion of the CNS to induce acute brain injury and conversion to a microglia-like sub-inflammatory state. This is a significant subject because little is known about monocyte-micorglia interactions/transformation in the neomnatal brain and pro-inflammatory monocyte-derivatives may increase the brain susceptibility to kindling and convulsion after injury. Further, understanding how infiltrating monocytes contribute to injury may lead to novel therapeutic strategies. In this project, we will test the hypothesis that perinatal infection/hypoxic-ischemic injury promotes monocytes to invade the CNS and exert inflammatory and destructive actions. Later, some of these monocytes adopt the microglial identity but are locked-in in a pro-inflammatory state to raise the brain susceptibility to excitotoxins and the kindling to convulsions. We will test this hypothesis n two specific aims. Aim 1: Significance of invading monocytes in acute infection/HI brain injury of newborns. We will use bitransgenic CX3CR1-GFP: CCR2-RFP mice to label microglia and monocytes respectively, and subject these mice to endotoxin (LPS)-sensitized hypoxia-ischemia (HI) injury to test the influx of monocytes in neonatal brains and their expression of pro-inflammatory cytokines. Next, we will compare the responses by hemizygous CCR2RFP/+ and homozygous CCR2RFP/RFP mice (equivalent to CCR2-null and lacking the receptor for Monocyte Chemoattractant Protein-1) to LPS/HI insult, as well as the benefits of anti-CCR2 antagonists to assess the pathogenic role of monocytes. Aim 2: Developmental consequences of monocyte influx after perinatal infection/HI brain injury. We will intravenously transfer actin-GFP/CCR2-RFP monocytes into LPS/HI-injured murine pups and follow their developmental outcomes. Based on the GFP-revealed morphology and double labeling with anti-cytokine antibody, we wiil test whether monocytes convert to microglia and maintain a pro-inflammatory state in adult brains. Next, we will use CCR2-mutant mice and a selective CCR2 antagonist (RS102895), to test whether or not monocyte-derived microglia sensitize the brain response to the kainic acid excitotoxicity and PTZ-induced kindling to convulsions.